Each year in the United States, five million patients get initial diagnosis of a heart attack, and nearly one million patients undergo coronary angioplasty, or other interventional procedures, to open or restore flow through stenosed vessels. Angiography is the standard method for assessing lesion severity, but it only provides an anatomic view of the lumen of the vessel, often in only one plane. Clinical benefits, as well as other benefits, would result if a real-time assessment of the functional severity of the lesion and its effect on blood flow were possible. A current method for attempting to acquire this information is the Doppler guidewire via which flow (or flow velocity) can be measured at the lesion. For reliable measurements, a catheter must be accurately positioned and must be stable during the entire data collection interval. This is difficult to do and, consequently, this method is not widely used. In addition, the necessary equipment is expensive and requires an elaborate training program for proficient use. A method involving direct measurement of pressure, rather than velocity, will have distinct advantages. Direct pressure measurements are easier to interpret, more familiar to medical personnel, require less expensive recording instruments and signal processing devices, and the position of the catheter is less critical. In addition, velocity measurements assess flow only through the lesion, while pressure measurements also assess the effects of collateral flow from other sources. This collateral flow can mediate the effect of the lesion in some cases. A direct-reading pressure catheter system can be used during angioplasty to monitor the progress and the immediate effects of the procedure on pressure distal to the lesion.
During angioplasty procedures, it is useful to be able to measure pressure distal to the lesions before, during, and after dilatation by the balloon. A procedure currently being investigated is the measurement of distal pressure during maximal vasodilatation. This is referred to as “functional flow reserve” and is a measure of the effect of the pressure drop across the lesion at maximal flow. This is currently measured through the lumen of the angioplasty catheter, but has limited fidelity, and can itself add to the severity of the lesion and the measured pressure drop. A narrow pressure sensor for direct pressure measurements was introduced in the U.S. market in February 1999, by RADI of Sweden called PressureWire™. The PressureWire™ sensor has a 360 micron diameter. However, there are several limitations with this sensor: (1) cost effectiveness, (2) mechanical characteristics, and (3) pressure measurement stability during angioplasty procedures. The current invention is related to a disposable sensor that reduces these limitations.
With the advent of the RADI PressureWire™, many studies have been conducted to determine the specific usefulness of such a device for diagnosis and an assessment of the effectiveness of the treatment during angioplasty. The high interest in such a device is demonstrated with over 20 papers presented about the RADI PressureWire™ at the ACC meeting held March, 2000 in Anaheim Calif. A new index, the Fractional Flow Reserve (FFR), defined as FFR=Pa/Pd (Pa=aortic pressure and Pd=distal coronary pressure), can be obtained by such a device and is now considered to be an accurate, quantitative and cost effective method for diagnosis and assessment. In particular, the method is effective for accurately determining the clinical significance of moderate stenoses. These are difficult to determine with current angiography procedures.
Presently the most common mass-produced disposable pressure sensors in the medical industry are silicon electronic devices with a typical size of several millimeters in diameter for the sensing area, usually used together with fluid-filled catheters as external pressure transducers. They are based on the piezoresistive or capacitive properties of silicon crystal and need complex circuitry for signal processing, drift compensation, and noise reduction before the information is made available to the medical personnel. These devices have an inherently high hysteresis and significant short-term creep (i.e., within a few hours) and thus need frequent re-calibration. They cannot easily perform static DC measurements. They also need to have a certain minimum size for the pressure-sensing mechanism to generate an adequate signal, so it is difficult to reduce the size down to the sub-millimeter region at a reasonable cost. The RADI PressureWire™ overcomes the size problem. However, it is an electronic sensor and the inherent problems described above remain, including a drift problem. In addition, the narrow (high impedance) cable must be adequately shielded to reduce RF interference. The desired feel (or stiffness) of the guidewire is therefore very difficult to achieve.
Fiber-optic sensors for direct pressure measurements are generally known in the art. Fiber-optic sensors are of a relatively simple design, have an inherently smaller potential size, and offer other advantages. A fiber-optic sensor is safe, involving no electrical connection to the body; because the primary signal is optical it is not subject to electrical interference, is very small and flexible, and can be included in catheters for multiple sensing. In addition, fiber-optic devices lend themselves well to existing mass production techniques.
U.S. Pat. No. 5,987,995 to the present assignee describes a fiber-optic pressure catheter that is suited to be low-cost and disposable. The sensor of the '995 patent includes a ribbon reflector, in contact with a polyurethane window, as the key sensing element that translates mechanical deformation, due to pressure, to an optical intensity variation of a signal beam. For some applications, the sensor of the '995 patent is undesirably large.
It would therefore be desirable to provide a pressure sensing system that is capable of providing a sufficient amount of deflection for the membrane in order to improve the accuracy of the device, increase the sterility of the system, and provide a means for adjusting the sensitivity so that consistent pressure readings are obtained if the sensor is disconnected from the light source and monitoring system.